Interloper

Terratus the planet has two satellites visible by the naked eye - the tide locked Terratus Grave and the rapidly orbiting Interloper. Scholars have long studied the effects of these two moons on the ocean currents, and a great deal of folklore concerns the origins of the moons.

Background
Notably, magic depends on the presence of the Interloper. If its orbit leads it in front of the Terratus Grave, any attempt at using magic that draws on the Grave will result in magical calamity.

Local customs
A common belief in the Empire is that Terratus Grave will finally set on the horizon when Kyros' rule ends, whereas those of the Tiers insist the moons are older than the Oldwalls and will outlive even Kyros.